Cursing, John steps gingerly over a stack of photographs of a particularly unpleasant crime scene. Sherlock studies him curiously as he makes his way into the kitchen. He looks into six or seven mugs, sighing at each one, until he gets to the last one, at which point he cringes and swears again. He's found the one with the toes in it, then.

Sherlock turns over on the sofa, eager to avoid eye contact and the ensuing argument. Unfortunately, John's having none of that.

"Sherlock, turn over or I'm dumping these toes on you. Which I'm sure will ruin the validity of whatever the hell it is you're doing with them."

Groaning, Sherlock turns over again, so he's still lying on the sofa but John can see his face now.

"Please don't, John. It's for an experiment."

"Of course it is. Is the rest of the bloody mess in here also part of an experiment? Some kind of study as to how much filth the average English flatmate can handle before going absolutely fucking postal?"

It's not a conscious decision to leave the communal areas of the flat a disaster area, not really. It's just that there's so much more important stuff going on inside of Sherlock's head that when it comes time to tidy up, he just can't be bothered.